1. Field of the Invention
This invention relates generally to nucleic acid sequences encoding polypeptides that are associated with abiotic stress responses and abiotic stress tolerance in plants. In particular, this invention relates to nucleic acid sequences encoding polypeptides that confer drought, cold, and/or salt tolerance to plants.
2. Background Art
Abiotic environmental stresses, such as drought stress, salinity stress, heat stress, and cold stress, are major limiting factors of plant growth and productivity. Crop losses and crop yield losses of major crops such as soybean, rice, maize (corn), cotton, and wheat caused by these stresses represent a significant economic and political factor and contribute to food shortages in many underdeveloped countries.
Plants are typically exposed during their life cycle to conditions of reduced environmental water content. Most plants have evolved strategies to protect themselves against these conditions of desiccation. However, if the severity and duration of the drought conditions are too great, the effects on development, growth, and yield of most crop plants are profound. Continuous exposure to drought conditions causes major alterations in the plant metabolism which ultimately lead to cell death and consequently yield losses.
Developing stress-tolerant plants is a strategy that has the potential to solve or mediate at least some of these problems. However, traditional plant breeding strategies to develop new lines of plants that exhibit resistance (tolerance) to these types of stresses are relatively slow and require specific resistant lines for crossing with the desired line. Limited germplasm resources for stress tolerance and incompatibility in crosses between distantly related plant species represent significant problems encountered in conventional breeding. Additionally, the cellular processes leading to drought, cold, and salt tolerance in model drought- and/or salt-tolerant plants are complex in nature and involve multiple mechanisms of cellular adaptation and numerous metabolic pathways. This multi-component nature of stress tolerance has not only made breeding for tolerance largely unsuccessful, but has also limited the ability to genetically engineer stress tolerant plants using biotechnological methods.
Drought stresses, cold stresses, and salt stresses have a common theme important for plant growth and that is water availability. As discussed above, most plants have evolved strategies to protect themselves against conditions of desiccation; however, if the severity and duration of the drought conditions are too great, the effects on plant development, growth and yield of most crop plants are profound. Furthermore, most of the crop plants are very susceptible to higher salt concentrations in the soil. Because high salt content in some soils results in less water being available for cell intake, high salt concentration has an effect on plants similar to the effect of drought on plants. Additionally, under freezing temperatures, plant cells lose water as a result of ice formation that starts in the apoplast and withdraws water from the symplast. A plant's molecular response mechanisms to each of these stress conditions are common, and ion transporters play an essential role in these molecular mechanisms.
Common damage from different stresses such as drought, salinity, and cold stress, appears to be mostly due to dehydration (Smirnoff, 1998, Curr. Opin. Biotech. 9:214–219). Drought (water stress)-tolerant and -sensitive plants can be clearly distinguished by the dramatic accumulation of ions and solutes in tolerant plants that leads to osmotic adjustments (Bohnert H. J and Jensen. R. G., 1996, TIBTECH 14:89–97). Drought and high salt conditions may interact with mineral nutrition in a number of ways as a consequence of (1) reduced transport of ions through the soil to the roots; and/or (2) modified uptake of ions by the roots.
Potassium is particularly important in plants not only as a nutrient, but also as an osmoticum. Potassium can make a 30–50% contribution to water potential, particularly in older leaf tissues (Munns R. et al., 1979, Aust. J. Plant Physiol. 6:379–389). After prolonged drought in the field, potassium accumulates in leaves of ryegrass and barley, and could have a role in osmotic adjustment. In addition, potassium plays a key role in the opening of the stomata. Because potassium is lost from the guard cells (Ehret, D. L. and Boyer, J. S., 1979, J. Exper. Bot. 30:225–234), a reduced supply of potassium reduces stomatal conductance to CO2 much more than it reduces internal conductance (Terry, N. and Ulrich, A., 1973, Plant Physiol. 51:783–786).
Plant roots can absorb potassium over more than a 1000-fold concentration range, and the concentration dependence of potassium uptake by roots has complex kinetics, suggesting the presence of multiple potassium uptake systems. Gene families encoding inward-rectifying K+ channels have been identified in several plant species. The AKT1 K+ channel gene is predominantly expressed in roots and genetic analysis indicates that the AKT1 channel mediates the uptake of K+ in both the micromolar and millimolar ranges (Hirsch, R. H. et al., 1998, Science 280:918–921). Active transporters also participate in K+ uptake, and several candidate genes encoding energized transporters have been identified (Hirsch, R. E and Sussman, M. R., 1999, TIBTECH 17:356–361).
Little is known about the role of zinc ions in stress tolerance in plants. Zinc ions are often localized to the leaf vacuoles where they could have a role as an osmoticum. Zinc is also a co-factor for free-radical scavenging enzymes like Cu/Zn super-oxide dismutase. Zinc, like other metals, is often indirectly related to stress tolerance. An increase in the activity of zinc-chelating enzymes has been implicated in stress tolerance. However, there is no evidence to date that zinc transporters play a role in stress tolerance.
Although some genes that arc involved in stress responses in plants have been characterized, the characterization and cloning of plant genes that confer stress tolerance remains largely incomplete and fragmented. For example, certain studies have indicated that drought and salt stress in some plants may be due to additive gene effects, in contrast to other research that indicates specific genes are transcriptionally activated in vegetative tissue of plants under osmotic stress conditions. Although it is generally assumed that stress-induced proteins have a role in stress tolerance, direct evidence is still lacking, and the functions of many stress-responsive genes are unknown.
There is a need, therefore, to identify genes expressed in stress tolerant plants that have the capacity to confer stress resistance to its host plant and to other plant species. Newly generated stress tolerant plants will have many advantages, such as increasing the range in which crop plants can be cultivated by, for example, decreasing the water requirements of a plant species.